We investigated Contrast enhancing lesions (CELs) on MRI in 30 multiple sclerosis (MS) patients. IFN-beta-1b decreases the formation of CELs. However, the ability of IFN-beta-1b to reduce the size of CELs arising during therapy has not been extensively investigated. MS patients were followed for a 3-month pre-therapy phase then for a 6-month therapy phase during which treatment with IFN-beta-1b at a dosage of 250 microg subcutaneously injected every other day was employed. Each patient underwent monthly clinical and MRI examinations. The average number and total lesion volume of CELs visible during the therapy phase were significantly lower than the number and total lesion volume of CELs observed in the pre-therapy phase. However, there was no significant reduction between pre-therapy and therapy phases in the mean size of individual lesions arising during the respective phases. Since size of CELs has been related to severity of tissue damage, the lack of size decrease during therapy suggested a limited therapeutic effect of IFN-beta-1b if a blood-brain barrier breakdown has occurred and ultimately results in long term white matter damage.